Microwave packages and modules, such as Integrated Microwave Assemblies (IMAs), typically include iron-based alloys or aluminum composites and plated layers that contain hydrogen as a result of the manufacturing process. Usually, the hydrogen will outgas and not present a problem. However, in a hermetically sealed package, hydrogen can reach as high as a few percent. Hydrogen causes degradation, or poisoning, of some types of GaAs devices over a wide range of partial pressures, with such poisoning resulting in a sudden and dramatic change in device electrical performance after several hundred to several thousand hours of hydrogen exposure at elevated temperatures.
Hydrogen poisoning of high electronic mobility transistor (HEMT) and metal semiconductor field effect transistor (MESFET) devices used in the above mentioned IMAs is a major reliability issue. Many materials used in the manufacturing and/or assembly of IMAs such as, for example, Ni and Au plated Kovar™ or A40, and even some RF absorber materials, are known to outgas hydrogen. A number of conventional options for dealing with this problem include eliminating or minimizing the hydrogen source, changing device technology, using an in-package hydrogen getter, and compensating for device electrical changes through circuit design.
Eliminating or minimizing the hydrogen source can be accomplished to a certain extent by vacuum baking the package parts or choosing low hydrogen soluble materials. However, it is difficult to completely eliminate all hydrogen-bearing materials. An alternate approach is to modify the device technology with the use of a gate metal, such as Tiw that is more hydrogen insensitive. While this may be an adequate approach for a relatively new technology, changing processes in relatively mature industries, which have a heritage of use and field experience, is not usually favorable.
A more desirable method is to use an in-package hydrogen getter to reduce hydrogen partial pressures to safe levels. The use of hydrogen getters in semiconductor packaging is common, and there are several commercially available hydrogen getters that can be employed in microwave packaging.
RF instability due to reflections from the metal IMA chassis and the back-scattering of microwaves inside the IMA is another major problem in microelectronic technology today, especially in high frequency/high gain MMIC devices. Microwave absorbers, also known as lossy materials or high loss tangent, help eliminate radio frequency (RF) instabilities due to microwave reflections inside the cavities of the IMAs of the microwave devices they support. Such reflections within the IMA chassis lead to wave—wave microwave iterations, which result in unwanted microwave signals. The absorbers function much like a band-pass filter as they absorb unwanted signals, and pass desired signals through the IMA. Commercially available microwave absorbers include such brands as Eccosorb™.
Microwave absorbers achieve absorption by significantly reducing the reflective properties of the metal structures of the IMA due to the flow of microwave currents on the surface where they are placed, dampening the cavity resonances of the microwave modules. This results in the isolation, attenuation and/or modification of the radiating patterns of the microwave devices, eliminating undesired RF instability.
While hydrogen getters and microwave absorbers are commercially available, they are individual components and require a larger footprint inside the IMAs. Examples of material used in commercially available hydrogen getters/absorbers include palladium oxides and titanium platinum or titanium palladium metals. The trend in microwave technologies today is to place a growing number of microwave devices in enclosures with ever decreasing dimensions, requiring increased component protection in a limited space. It is often impractical or impossible to add the required protection in the chassis with microwave absorbers and hydrogen getters in their current format due to limited space in IMA designs.
Furthermore, there is an additional cost for purchasing two separate products for the protection of microwave circuitry, raising the cost to both manufacturers and end users alike.
Additionally, due to space constraints, separate components will have a decreased capacity to limit hydrogen and microwave degradation relative to more appropriately sized components.